The present invention relates to an image printing apparatus and, more particularly, to an image correction system for correcting density nonuniformity between a plurality of heads or density nonuniformity between main-scanned images by a print head.
In recent years, image printing using inkjet print heads and thermal print heads as image printing apparatuses has rapidly being spread with the development of electronic video devices. Particularly, image printing using a high-speed printing multihead have been developed.
An image printing apparatus using a multihead prints an image by one head formed by adhering a plurality of print heads each having a plurality of nozzles.
FIG. 17A shows an ideal image printing state by the multihead printing apparatus.
In FIG. 17A, upper black dots form an image printed by a head H1, whereas lower blank dots form an image printed by a head H2.
To form an adhered head so as to print a clear image, as shown in FIG. 17A, the heads H1 and H2 must be aligned with a very high precision.
However, if the heads H1 and H2 are excessively apart from each other at different angles, as shown in FIG. 17B, a blank stripe appears in an image corresponding to the joint between the heads H1 and H2, or images printed by the upper and lower heads geometrically change.
If the heads H1 and H2 overlap each other at different angles, as shown in FIG. 17C, a black stripe appears in an image printed at the joint between the heads H1 and H2.
To prevent this, the conventional multihead printing apparatus requires a long time for adjusting the head position, resulting in a high-cost multihead printing apparatus.
In the conventional multihead printing apparatus, even if heads are aligned with a high precision, printed images may overlap or be omitted at the boundary between one main-scanned image and the next main-scanned image owing to decentering of a roller for feeding a paper sheet serving as a printing medium, a slip of a paper sheet, and the like.
In the conventional multihead printing apparatus, the head position may change over time in accordance with the environment (temperature and humidity) of the printing apparatus and the like. For this reason, the image quality must be properly kept constant in accordance with the instantaneous state of printing.
To solve these problems, Jpn. Pat. Appln. KOKAI Publication No. 61-121658 discloses a printing apparatus wherein previous and current main-scanned images partially overlap each other, and an image is printed based on a predetermined pattern formed such that the previous and current scanned images become complementary to each other in this overlapping region.
The prior art disclosed in Jpn. Pat. Appln. KOKAI Publication No. 61-121658 can avoid to a certain degree a black or blank stripe generated in an image corresponding to the head joint. However, the heads must still be aligned with a high precision.
In addition, Jpn. Pat. Appln. KOKAI Publication No. 61-121658 does not disclose any multihead printing apparatus itself formed by adhering a plurality of heads.